Case Studies

Whiteness Indices and UV Standards

The evaluation of the whiteness of a product is dependent upon the materials and the application in which it is used. Natural materials, for instance, cotton or wool, tend to yield a yellowish tint, so the industry will make modifications to the materials to compensate for this effect. A yellowish tint in a product is most often seen as a quality flaw, e.g., yellowing due to aging or dirt, and businesses will attempt to make the appearance of their products more white.

Bleaching is a process that chemically removes colors from materials and results in a more uniform spectral reflectance. Optical brightening agents, or fluorescents, are also used to compensate for the absorbance of yellowish products. This creates a “whiter than white” appearance with the help of the fluorescents.

Optical brighteners absorb energy from the electromagnetic spectrum in the non-visible UV area (mostly below 400nm) and emit that energy in a wider spectrum than was absorbed in the range between 400-480nm. This results in reflectance curves that may rise higher than 100% between 400-480nm, making the material appear slightly bluish. As the eye will judge slightly bluish materials of otherwise uniform reflectance as brighter than the ideal reflecting diffuser, these colorants are a very common way of adding additional whiteness to products, and are often used in paper and textiles. “White” is not compliant to what we judge “color,” as both sensations are independent from each other.

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